Fill a watertight bowl or other glass container with lukewarm water as high as you'd like the petunia blossoms to float. If you're using a wide glass vase, for example, the petunias could float in the middle of the vase instead of near the top. If you're using a clay bowl, the flowers could float near the top so they're visible.
Cut an odd number of blossoms in full bloom from their stems. This looks more natural in arrangements than even numbers. Leave a 1/2-inch of each stem to help balance the flower in the water.
Leave the leaves that remain on the top 1/2-inch of stem if they come above the water line; otherwise, cut or gently pull the leaves off.
Set the cut flowers in the water, allowing enough room for them to float freely and not be bunched together on the water's surface.